Plug Test
WARNING: This page is about a different car, the 2002 Ford Ranger. However, it is still accessible from the selected car via links, so may be relevant.
Check for proper coolant level. Start engine and turn heater on. When engine reaches normal operating temperature, feel heater outlet hose. It should be hot. If outlet hose is not hot, heater core may have an air pocket, may be plugged, or thermostat may not be working properly. If outlet hose is hot, perform appropriate pressure test.